Na hora que cria a transferência, ela não aparece na lista.

import 'package:flutter/material.dart';

void main() => runApp(ByteBankapp());

class ByteBankapp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: ListaTransferencias(),
      ),
    );
  }
}

class ListaTransferencias extends StatefulWidget {
  final List<Transferencia> _transferencias = [];


  @override
  State<StatefulWidget> createState() {

   return ListaTransferenciaState();
  }
}

class ListaTransferenciaState extends State<ListaTransferencias>{

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(title: Text('Transferências')),
        body: ListView.builder(
          itemCount: widget._transferencias.length,
          itemBuilder: (context, indice) {
            final transferencia = widget._transferencias[indice];
            return ItemTransferencia(transferencia);
          },
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            final Future<Transferencia?> future = Navigator.push(context, MaterialPageRoute(builder: (context) {
              return FormularioTransferencia();
            }));
            future.then((transferenciaRecebida) {
              debugPrint('Chegou no then do Future');
              debugPrint('$transferenciaRecebida');
              widget._transferencias.add(transferenciaRecebida!);
            });
          },
          child: Icon(Icons.add),
        ));
  }

}

class ItemTransferencia extends StatelessWidget {
  final Transferencia _transferencia;

  ItemTransferencia(this._transferencia);

  @override
  Widget build(BuildContext context) {
    return Card(
      child: ListTile(
        leading: Icon(
          Icons.monetization_on,
        ),
        title: Text(_transferencia.valor.toString()),
        subtitle: Text(_transferencia.numeroConta.toString()),
      ),
    );
  }
}

class FormularioTransferencia extends StatelessWidget {
  final TextEditingController _controladorCampoValor = TextEditingController();
  final TextEditingController _controladorCampoNumeroConta =
      TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('Criando Transferência'),
        ),
        body: Column(
          children: <Widget>[
            Editor(
              controlador: _controladorCampoNumeroConta,
              rotulo: 'Número da Conta',
              dica: '0000',
              icone: Icons.account_balance,
            ),
            Editor(
                controlador: _controladorCampoValor,
                rotulo: 'Valor',
                dica: '0.00',
                icone: Icons.monetization_on),
            ElevatedButton(
              onPressed: () => _criaTransferencia(context),
              child: Text('Confirmar'),
            ),
          ],
        ));
  }

  void _criaTransferencia(BuildContext context) {
      final int? numeroConta =
        int.tryParse(_controladorCampoNumeroConta.text);
    final double? valorTransferencia =
        double.tryParse(_controladorCampoValor.text);
    if (numeroConta != null && valorTransferencia != null) {
      final transferenciaCriada =
          Transferencia(valorTransferencia, numeroConta);
      debugPrint('Criando Transferência');
      debugPrint('$transferenciaCriada');
      Navigator.pop(context, transferenciaCriada);
    }
  }
}

class Editor extends StatelessWidget {
  final TextEditingController controlador;
  final String rotulo;
  final String dica;
  final IconData icone;

  Editor(
      {required this.controlador,
      required this.rotulo,
      required this.dica,
      required this.icone});

  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: const EdgeInsets.all(16.0),
      child: TextField(
        controller: controlador,
        style: TextStyle(fontSize: 24),
        decoration: InputDecoration(
          icon: Icon(icone),
          labelText: rotulo,
          hintText: dica,
        ),
        keyboardType: TextInputType.number,
      ),
    );
  }
}

class Transferencia {
  final double valor;
  final int numeroConta;

  Transferencia(this.valor, this.numeroConta);

  @override
  String toString() {
    return 'Transferencia{valor: $valor, numeroConta: $numeroConta}';
  }
}